1. Field of the Invention
The present invention relates to a Multiple Input Multiple Output (MIMO) system. More particularly, the present invention relates to an apparatus and method for reducing an inter-cell interference in a MIMO system.
2. Description of the Related Art
Recently, research on a technique for controlling an inter-cell interference through inter-Base Station (BS) coordination has been conducted. The inter-BS coordination technique is called Coordinated Multiple Point Transmission/Reception (CoMP) in the 3rd Generation Partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) standard and is called Multi-cell Multiple Input Multiple Output (MIMO) in the Institute of Electrical and Electronics Engineers (IEEE) standard 802.16m. Although the following description is given of the CoMP technique with reference to the 3GPP standardization group, it is also applicable to the multi-cell MIMO technique of the IEEE standard 802.16m.
The CoMP technique is divided into a Coordinated scheduling and/or Beamforming (CB) technique and a Joint Processing/transmission (JP) technique. Similar to a conventional single-cell MIMO technique, the CB technique transmits traffic data for one User Equipment (UE) from one transmitting node (i.e., a BS or a cell). Thus, the CB technique does not require inter-cell packet sharing and cannot use a Signal-to-Noise Ratio (SNR) enhancement technique. On the other hand, the JP technique transmits traffic data for one UE from two or more transmitting nodes. Thus, the JP technique can enhance an SNR by means of the SNR enhancement technique and requires inter-cell packet sharing.
An example of the CB technique is a Precoding Matrix Information (PMI) restriction technique. In the PMI restriction technique, a UE located at a cell boundary requests a serving BS to prevent a precoding vector from being used by an adjacent BS if the UE receives a strong interference from an adjacent cell due to the use of a precoder between adjacent cells. The adjacent BS does not use the precoding vector according to the request. Accordingly, the inter-cell interference decreases and thus the PMI restriction technique can enhance an SNR.
Examples of the JP technique include a Macro diversity technique and a multi-cell Zero-Forcing BeamForming (ZFBF) technique. The macro diversity technique transmits traffic data from multiple cells to only one UE. The macro diversity technique has poor frequency efficiency. However, because the macro diversity technique uses an interference channel as an effective signal channel, it can reduce the interference and increase the signal strength. The multi-cell ZFBF technique transmits traffic data from multiple cells to multiple UEs simultaneously and uses downlink channel information to reduce the interference between the UEs. The multi-cell ZFBF technique is very similar to a single-cell ZFBF technique. However, due to an addition of a backhaul and a multi-cell concept, the multi-cell ZFBF technique is difficult to implement and its throughput may depend on the backhaul characteristics.
As described above, the CoMP technique increases the system complexity and cannot secure performance improvement in an actual system. More specifically, the CoMP technique has the following three problems.
Firstly, the CoMP technique is problematic in terms of system operation. A conventional CoMP technique requires a fast operation in comparison with the conventional single-cell MIMO technique. For example, the CoMP technique must select target UEs and manage the selected UEs in a separate manner, which increases the complexity of a scheduler and reduces resource utilization efficiency. Also, except for the macro diversity technique, the CoMP is applicable only to a closed-loop MIMO technique. Therefore, a moving UE cannot benefit from noise reduction.
Secondly, if the conventional CoMP technique is applied to the system, cells participating in a coordination must be grouped into clusters for scheduling and transmission. In this case, because the number of cells included in a cluster is restricted, only a portion of a multi-cell interference can be controlled. Thus, although the conventional CoMP technique is applied, the throughput may be restricted by the interferences between uncontrolled remaining cells, which are received from outside of the cell cluster. Also, there is an increased probability that scheduling complexity may increase due to a cluster-by-cluster multi-cell scheduling.
Thirdly, the CoMP technique has a throughput restriction problem due to a backhaul. The conventional CoMP technique must exchange coordination information through backhaul for inter-BS coordination. In this case, the coordination level can increase according to the amount of exchanged coordination information, but the throughput may degrade due to the time delay taken for an inter-BS information exchange. Similarly, the conventional CoMP technique requires a high backhaul overhead and is very sensitive to backhaul latency. Therefore, improvement of the throughput versus the overhead may be small if there is not a good backhaul.
Therefore, a need exists for an apparatus and method for reducing an inter-cell interference while having low scheduling complexity and low backhaul dependence.